Semiconductor light emitting device and method for producing the same

ABSTRACT

A light emitting device employing gallium nitride type compound semiconductor which generates no crystal defect, dislocation and can be separated easily to chips by cleavage and a method for producing the same are provided. As a substrate on which gallium nitride type compound semiconductor layers are stacked, a gallium nitride type compound semiconductor substrate, a single-crystal silicon, a group II-VI compound semiconductor substrate, or a group III-V compound semiconductor substrate is employed.

This is a Division of application Ser. No. 10/127,810, filed Apr. 23,2002, now U.S. Pat. No. 6,680,492; which is a Division of applicationSer. No. 09/569,300, filed May 11, 2000, now U.S. Pat. No. 6,376,866;which is a Division of application Ser. No. 09/149,435, filed Sep. 08,1998, now U.S. Pat. No. 6,087,681; which is a Division of ApplicationSer. No. 08/517,121, filed Aug. 21, 1995, now U.S. Pat. No. 5,838,029.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor light emitting deviceand a method for producing the same, and more particularly, asemiconductor light emitting device comprising a gallium nitride typecompound semiconductor for emission of blue light and a method forproducing the same.

Such a gallium nitride type compound semiconductor is (1) asemiconductor comprising a compound of Ga of group III element and N ofgroup V element, or (2) a semiconductor which comprises a GaN compoundin which a part of Ga is substituted by other group III elements such asAl or In and/or a part of N is substituted by other group V elementssuch as P or As.

The semiconductor light emitting devices include light emitting diodes(hereinafter referred to as “LED”) having pn junctions or doubleheterojunctions such as pin junctions, super-luminescent diodes(hereinafter referred to as “SLD”), and semiconductor laser diodes(hereinafter referred to as “LD”).

Although conventional blue LEDs are lower in luminance than red or greenones and disadvantageous for practical use, they have been improved byusing gallium nitride type compound semiconductor and more specifically,doping an amount of Mg thus forming a p-type semiconductor layer with alow resistance and are now available for new applications.

A conventional gallium nitride LED has a structure shown in FIG. 7. Itis fabricated by applying gaseous forms of metal organic compounds suchas trimethylgallium (TMG) and ammonia. (NH₃) together with a carrier ofH₂ gas to a single-crystal substrate 51 of sapphire (Al₂O₃) at a lowtemperature of 400° C. to 700° C. using a metal organic chemical vapordeposition (MOCVD) method to form a low-temperature buffer layer 54 of,approximately 0.01 to 0.2 micrometer thick, comprising GaN, and applyingthe gaseous forms of the same materials at a high temperature of 700° C.to 1200° C. to form a high-temperature buffer layer 55, approximately 2to 5 micrometers thick, comprising n-type GaN which is identical in thechemical composition to the layer 54.

A gaseous form of trimethylaluminum (TMA) is then added to theprescribed materials to deposit an n-type cladding layer 56 of,approximately 0.1 to 0.3 micrometer thick, comprising Al_(x)Ga_(1−x)N(where 0<x<1) for creating a double heterojunction. Those n-type layersare prepared by depending on the fact that gallium nitride type compoundsemiconductor materials can be made n-type without addition of anyn-type impurities or by simultaneous application of SiH₄ gas.

Then, the same materials including a less amount of Al and a more amountof In than in the cladding layers are deposited to form an active layer57 which is comprising, for example, Ga_(y)In_(1−y)N (where 0<y≦1) andlower in band gap energy than the cladding layers.

Also, a p-type impurity of Mg or Zn in the form of a metal organiccompound gas of e.g. bis(cyclopentadienyl)magnesium (CP₂Mg) ordimethylzinc (DMZn) is added to the same gaseous materials as of then-type cladding layers in a reaction tube to form a p-type claddinglayer 58 comprising p-type Al_(x)Ga_(1−x)N.

Furthermore, the same gaseous materials are applied for vapor depositionof a p-type GaN cap layer 59.

Whole surfaces of growth layers of the semiconductor material is thencoated with a protective layer of e.g. SiO₂ and the like and annealedfor approximately 20 to 60 minutes at a temperature ranging from 400° C.to 800° C., allowing both the p-type cap layer 59 and the p-typecladding layer 58 to be activated. After the protective layer isremoved, a resist pattern is applied for assigning n-type electrodes.When the semiconductor layers are subjected to dry etching by chlorineplasma atmosphere, desired regions of the n-type GaN high-temperaturebuffer layer 55 are exposed as shown in FIG. 7. Finally, two electrodes59 and 60 are formed by sputtering of a metal film such as Au or Al. Thesemiconductor layers are then diced to LED chips.

As understood, a conventional semiconductor light emitting device usingthe gallium nitride type compound semiconductor material has at backside a sapphire substrate made of an insulating material. For formingelectrodes on the back side, it is hence needed to use etching or othercomplicated processing method.

Although the sapphire substrate withstands a high temperature and iseasily bonded to any type of crystal surface, the sapphire is verydifferent from the gallium nitride semiconductor material in latticeconstant, 4.758 (sapphire substrate) angstrom to 3.189 (gallium nitridetype semiconductor crystal) angstrom, and also, in coefficient ofthermal expansion. The difference in lattice constant may result incrystal defect or dislocation in the buffer layer stacked on thesapphire substrate as denoted by A in FIG. 8. If the crystal defectpropagates to the single-crystal gallium nitride type compoundsemiconductor layers which are stacked on the buffer layer and act asoperating layers, operating region is declined and also opticalcharacteristics of the semiconductor layers degrade.

In addition, the sapphire substrate is hardly cleft and it is thus noteasy to produce semiconductor light emitting device chips by cleavingabove-mentioned structure of the semiconductor layers. It is said thatthe conventional semiconductor layer structure described above is notappropriated for producing particular devices such as semiconductorlaser devices in which two opposite sides are required to be mirrorsurfaces which are parallel with each other at high accuracy. It is alsohard to process the sapphire substrate which may thus be processed withmuch difficulty.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedsemiconductor light emitting device and a method for producing the samewherein the above disadvantages are eliminated and the generation ofundesirable artifacts including the crystal defect and dislocation whichmay result from mismatch in lattice constant or thermal expansioncoefficient are minimized.

It is a further object of the present invention to provide asemiconductor light emitting device and a method for producing the same,the semiconductor light emitting device having multilayer structurewherein processing like as separating wafers to chips easily bycleaving, for example, is easily performed. Consequently, galliumnitride type compound semiconductor according to the present inventionenables to obtain mirror surfaces as end surfaces by cleaving for asemiconductor light emitting device which needs, like semiconductorlaser, two mirror surfaces which are parallel with each other as endsurfaces of the device.

A semiconductor light emitting device according to the first aspect ofthe present invention in order to achieve the object comprises asingle-crystal silicon substrate, an insulating layer formed on thesingle-crystal silicon substrate, and gallium nitride type compoundsemiconductor layers provided on the insulating layer.

It is preferable to employ a single-crystal silicon substrate of which(111) crystal plane is a principal plane, since an insulating layer ofwhich lattice matching with gallium nitride type compound semiconductorlayer at an interface between the gallium nitride type compoundsemiconductor substrate and the insulating layer is appropriate can beobtained.

The gallium nitride type compound semiconductor layers may be aplurality layers including a p-type layer and an n-type layer and anactive layer for emission of light. This structure is preferable so asto provide the light emitting device. The gallium nitride type compoundsemiconductor layers comprises a buffer layer, a lower cladding layer,an active layer, an upper cladding layer, and a cap layer.

The buffer layers are made of n-type GaN, the lower cladding layer ismade of n-type Al_(x)Ga_(1−x)N (0<x<1), the active layer is made ofGa_(n)In_(1−n)N (0<n≦1), the upper cladding layer is made of p-typeAl_(x)Ga_(1−x)N (0<x<1), and the cap layer is made of p-type GaN, thusthe light emitting device with double hetero structure can be provided.

A method for producing the semiconductor light emitting device accordingto the first aspect of the present invention comprises the steps of:

(a) forming an insulating layer on a single-crystal silicon substrate;

(b) forming a gallium nitride type compound semiconductor layer as abuffer layer on the insulating layer;

(c) stacking on the buffer layer in sequence a lower cladding layer, anactive layer, an upper cladding layer, and a cap layer, these layersbeing made of gallium the nitride type compound semiconductor;

(d) exposing a predetermined surface of the buffer layer by etchingperpendicularly to the single-crystal silicon substrate;

(e) forming electrodes on both the cap layer and the predeterminedsurface of the buffer layer exposed by the etching treatment in step(d), whereby obtaining a semiconductor wafer having multilayerstructure; and

(f) separating the semiconductor wafer to chips by dicing or bycleaving.

According to the first aspect of the present invention, the insulatinglayer of, for example, silicon nitride or aluminum oxide is deposited onthe single-crystal silicon substrate and then, single-crystal galliumnitride type compound semiconductor layers which are operating layersare grown on the gallium nitride type compound semiconductor bufferlayers which are on the insulating layer. As a result, the depositedlayers on the substrate are similar to one another in lattice constantand coefficient of thermal expansion, thus having less chance ofundesirable lattice defect and dislocation.

The buffer layer is provided for preventing crystal defect generated bylattice mismatch between the gallium nitride type compound semiconductorlayers and the insulating layer on the single-crystal semiconductorsubstrate from extending across the single-crystal gallium nitride typecompound semiconductor layers which are operating layers and fromgenerating other defect or dislocation. The buffer layer may have adouble layer structure including the low-temperature buffer layer andthe high-temperature buffer layer to minimize and relax the latticemismatch efficiently.

Also, since the single-crystal semiconductor layers of the buffer layerand the cladding layer, thickness of each of the buffer layer and thecladding layer being at least one micrometer, are identical to eachother in chemical composition, their cleft edges are highly planar incleft surface, thereby mirror surfaces being easily obtained.

To achieve the foregoing object, according to the second aspect of thepresent invention, a gallium nitride type compound semiconductorsubstrate is employed as the semiconductor substrate and gallium nitridetype compound semiconductor layers are stacked on the substrate.

The semiconductor light emitting device according to the second aspectof the present invention comprises gallium nitride type compoundsemiconductor layers stacked on the gallium nitride type compoundsemiconductor layers.

A method for producing a semiconductor light emitting device accordingto the second aspect of the present invention comprises the steps of:

(g) growing a gallium nitride type compound semiconductor layer on asingle crystal semiconductor substrate;

(h) removing the single-crystal semiconductor substrate; and

(i) growing single-crystal gallium nitride type compound semiconductorlayers including at least both an n-type layer and a p-type layer, onthe single-crystal gallium nitride type compound semiconductor layer,with utilizing the gallium nitride type compound semiconductor layer asa new substrate.

It is preferable that the single-crystal semiconductor substrate is madeof one member of selected from the group consisting of GaAs, GaP, InPand Si and has a (111) crystal plane, for optical and electricalcharacteristics of the gallium nitride type compound semiconductorlayers which are formed thereon.

It is also preferable that the step (g) of growing the gallium nitridetype compound semiconductor layer on the single-crystal semiconductorsubstrate may be implemented by forming the low-temperature buffer layerof the gallium nitride type compound semiconductor layer on thesingle-crystal semiconductor substrate at low temperature of 400° C. to700° C. and forming the gallium nitride type compound semiconductorlayer at higher temperature of 700° C. to 1200° C. so that thelow-temperature buffer layer relaxes the lattice mismatch between thesubstrate and the low-temperature buffer layer and prevents crystaldefect or dislocation.

More preferably, before the step (i) of growing the gallium nitride typecompound single-crystal semiconductor layers, the low-temperature bufferlayer of the gallium nitride type compound semiconductor is formed atlow temperature of 400° C. to 700° C. and then, the high-temperaturebuffer layer of the gallium nitride type compound semiconductor isformed at high temperature of 700° C. to 1200° C. and is followed by thegrowing of the single-crystal semiconductor layers of gallium nitridetype compound to minimize crystal defect or dislocation produced in thegallium nitride type compound semiconductor substrate.

The single-crystal gallium nitride type compound semiconductor layersincluding at least both the n-type layer and the p-type layer comprisethe n-type cladding layer, the active layer, and the p-type claddinglayer, these three layers forming a sandwich structure. In particular,the band gap energy of the active layer is smaller than that of then-type cladding layer or the p-type cladding layer. Also, the n-typecladding layer, the p-type cladding layer, and high-temperature bufferlayer and the gallium type nitride type compound semiconductor substrateare the same in chemical composition, thus providing light emittingdevice with high efficiency of light emission.

It is preferable that a semiconductor wafer on which the single-crystalgallium nitride type compound semiconductor layers is then cleft todesired chips, thus providing the mirror end surfaces.

According to the second aspect of the present invention, after thegallium nitride type compound semiconductor layer is grown on asingle-crystal semiconductor substrate, the single-crystal semiconductorsubstrate is removed and single-crystal gallium nitride type compoundsemiconductor layers which are operating layers are provided on thegallium nitride type compound semiconductor layer which is now utilizedas a new substrate. The semiconductor layers are similar to one anotherin lattice constant and coefficient of thermal expansion, thus havingless chance of undesirable lattice defect and dislocation.

A crystal defect in the gallium nitride type compound semiconductorlayer which is grown on the original single-crystal semiconductorsubstrate and is utilized as a new substrate may be generated by latticemismatch between the gallium nitride type compound semiconductor layerwhich is the new substrate and the single-crystal semiconductorsubstrate. The crystal defect also may extend across the single-crystalgallium nitride type compound semiconductor layers which are operatinglayers. Thereby other dislocation or crystal defect may be generated.The crystal defect or dislocation is however prevented by the presenceof the low-temperature buffer layer and high-temperature buffer layerprovided between the gallium nitride type compound semiconductor layersand the new substrate.

Since the single-crystal semiconductor layers of the buffer layer andthe cladding layer, thickness of each of the buffer layer and thecladding layer being at least one micrometer, are identical to eachother in chemical composition, their cleft edges are highly planar incleft mirror surface.

To achieve the foregoing object, the third aspect of the presentinvention by use of a group II-VI compound semiconductor substrate as asemiconductor substrate.

The semiconductor light emitting device according to the third aspect ofthe present invention comprises gallium nitride type compoundsemiconductor layers stacked on a II-VI compound semiconductorsubstrate.

The gallium nitride type compound semiconductor layers may be stacked ona top surface of the substrate comprising group VI atoms of the groupII-VI compound material so that the lattice matching at interface isdesirable.

The group II-VI compound semiconductor substrate may be made of ZnSe.When the group II-VI compound semiconductor substrate is made of ZnSe,the substrate does not absorb light with at least 470 nanometerwavelength. When the semiconductor substrate is made of ZnS, thesubstrate does not absorb light with at least 320 nanometer wavelength.As a result, the light emitting semiconductor device can be providedwhich has improved efficiency of light emission.

A method for producing the semiconductor light emitting device accordingto the third aspect of the present invention comprises the steps of:

(j) preparing the group II-VI compound semiconductor substrate;

(k) stacking a buffer layer of gallium nitride type compoundsemiconductor on a principal plane of the group II-VI compoundsemiconductor substrate;

(l) stacking on the buffer layer in sequence a lower cladding layer, anactive layer, an upper cladding layer, and a cap layer, these layersbeing made of the gallium nitride type compound semiconductor withmatching crystal lattice of each layer to one another;

(m) forming electrodes on both the top of the cap layer and the bottomof the group II-VI compound semiconductor substrate, whereby obtaining asemiconductor wafer having multilayer structure; and

(n) cleaving the semiconductor wafer to chips.

Preferably the step of forming the buffer layers comprises steps offorming a low-temperature buffer layer at low temperature and forming ahigh-temperature buffer layer at high temperature.

According to the third aspect of the present invention, the substrate onwhich gallium nitride type compound semiconductor layers are grown ismade of a group II-VI compound semiconductor such as ZnSe or ZnS. Thegallium nitride type compound semiconductor layers are hence similar tothe group II-VI compound semiconductor substrate in both latticeconstant and coefficient of thermal expansion, having less chance ofundesirable crystal defect and dislocation.

Also, the group II-VI compound semiconductor substrate is employed witha top surface comprising the group VI atoms of the group II-VI compoundsemiconductor substrate, thus decreasing the lattice mismatch to thegallium nitride type compound semiconductor layers. This allows anycrystal defect or dislocation to rarely occur into the single-crystalgallium nitride type compound semiconductor layers which are operatinglayers.

Since the single-crystal semiconductor layers of the buffer layer andthe cladding layer, thickness of each of the buffer layer and thecladding layer being at least one micrometer, are identical to eachother in chemical composition, their cleft edges are highly planar incleft surface, thereby mirror surfaces being easily obtained.

According to the fourth aspect of the present invention so as to achievethe prescribed object, a group III-V compound semiconductor substrate isused as a semiconductor substrate.

The semiconductor light emitting device according to the fourth aspectof the present invention comprises gallium nitride type compoundsemiconductor layers stacked on a group III-V compound semiconductorsubstrate.

It is preferable that the gallium nitride type compound semiconductorlayers are stacked on a top surface of the substrate comprising group Vatoms of the group III-V compound material, thus providing a desirablelattice matching at interface.

The group III-V compound semiconductor substrate is preferably made of amember selected from the group consisting of GaAs, InAs, GaP and InP.

A method for producing the semiconductor light emitting device accordingto the fourth aspect of the present invention comprises the steps of:

(o) preparing the group III-V compound semiconductor substrate;

(p) stacking a buffer layer of gallium nitride type compoundsemiconductor on a principal plane of the group III-V compoundsemiconductor substrate;

(q) stacking on the buffer layer in sequence a lower cladding layer, anactive layer, an upper cladding layer, and a cap layer, these layersbeing made of gallium nitride type compound semiconductor substrate,with matching crystal lattice of each layer to one another;

(r) forming electrodes on both the top of the cap layer and the bottomof the group III-V compound semiconductor substrate, whereby obtaining asemiconductor wafer having maltilayer structure;

(s) cleaving the semiconductor wafer to chips.

According to the fourth aspect of the present invention, the substrateon which gallium nitride type compound semiconductor layers are stackedis made of a group III-V compound material such as GaAs, InAs, GaP, orInP. The gallium nitride type compound semiconductor layers are hencesimilar to the group III-V compound semiconductor substrate in latticeconstant and coefficient of thermal expansion, having less chance ofundesirable crystal defect and dislocation. In particular, the thermalexpansion coefficient of above-mentioned group III-V compound materialis approximate to that of GaN as compared with conventional sapphiresubstrate, and distortion in the lattice will be minimized during theheating process in fabrication process.

Also, the substrate is provided with a plane corresponding to C plane ofthe sapphire and a top surface comprises the group V atoms of the groupIII-V compound semiconductor substrate, thus decreasing the latticemismatch to the gallium nitride type compound semiconductor layers. Thisallows any crystal defect or dislocation to rarely occur in thesingle-crystal gallium nitride type compound semiconductor layers whichare operating layers. The group V atoms of the group III-V compoundsemiconductor substrate on the top surface of the substrate promotenitrogenizing on the substrate thus producing optimum alignment ofcrystalline planes in the gallium nitride type compound semiconductorlayers.

Since the single-crystal semiconductor layers of the buffer layer andthe cladding layer, thickness of each of the buffer layer and thecladding layer being at least one micrometer, are identical to eachother in the chemical composition, their cleft edges are highly planarin cleft surface, therby mirror surfaces being easily obtained.

In addition, when the substrate is made of a proper material comprisinggroup III atoms which are founded in the buffer layer, distortionbetween the substrate and the buffer layer can be minimized.

BRIEF EXPLANATION OF THE DRAWINGS

FIGS. 1( a) to 1(f) are explanatory diagrams illustrating steps ofproducing a semiconductor light emitting device according to the presentinvention;

FIG. 2 is an explanatory cross sectional view of an LED according to thepresent invention;

FIGS. 3( a) to 3(e) are explanatory diagrams illustrating steps forproducing a semiconductor light emitting device according to the presentinvention;

FIG. 4 is an explanatory cross sectional view of an LED produced by themethod according to the present invention;

FIG. 5 is an explanatory cross sectional view of a semiconductor laserdevice according to the present invention;

FIG. 6 is an explanatory cross sectional view of an LED according to thepresent invention;

FIG. 7 is an explanatory cross sectional view of a conventional galliumnitride LED.

FIG. 8 illustrates dislocation in a buffer layer formed on aconventional sapphire substrate.

DETAILED DESCRIPTION EXAMPLE 1

This Example 1 shows an example according to the first aspect of thepresent invention wherein gallium nitride type compound semiconductorlayers are stacked to product on a single-crystal sillicon substrate asa semiconductor substrate on which an insulating layer made of silliconnitride and the like are provided.

The semiconductor laser and a method for producing the same will beexplained in detail referring to FIGS. 1( a) to 1(f).

The procedure of the method starts with removing an oxide film from asingle-crystal silicon substrate 21 having a (111) crystal plane as itsprincipal plane. Then, 500° C. to 900° C. of heat is applied under anatmosphere of nitrogen gas to nitrogenize a surface region of thesubstrate 21, forming an insulating layer 22 which comprises Si₃N₄, forexample, and has a thickness of approximately 1 to 5 nanometers, asshown in FIG. 1( a).

This is followed by introducing metal organic gas and impurity dopantgas for deposition of layers using MOCVD method as is the case withprior arts explained hereinbefore, as shown in FIG. 1( b). Moreparticularly, in sequence on the insulating layer 22, a low-temperaturebuffer layer 23 comprising n-type GaN having a thickness ofapproximately 0.01 to 0.2 micrometer, a high-temperature buffer layer 24comprising n-type GaN having a thickness of approximately 2 to 5micrometers at temperature ranging 700° C. to 1200° C., a lower claddinglayer 25 comprising n-type Al_(x)Ga_(1−x)N (0<x<1) having a thickness ofapproximately 0.1 to 0.3 micrometer, an active layer 26 comprisingnon-doped Ga_(n)In_(1−n)N (0<n≦1) having a thickness of approximately0.05 to 0.1 micrometer, which is smaller in band gap energy than thecladding layers, an upper cladding layer 27 comprising p-typeAl_(x)Ga_(1−x)N having a thickness of approximately 0.1 to 0.3micrometer, and a cap layer 28 comprising p-type GaN having a thicknessof approximately 0.3 to 2 micrometers are provided.

Then the surface of stacked semiconductor layers is coated at the topwith a resist layer 31 of approximately 0.3 to 3 micrometers inthickness having a pattern uncovering regions to be etched, as shown inFIG. 1( c). The pattern of the resist layer is arranged so that the sideof its opening 32 is substantially perpendicularly to the top surface ofthe substrate 21 or the stacked semiconductor layers.

Next, the stacked semiconductor layers is subjected to reactive ionetching process under an atmosphere of Cl₂ plasma gas where the activelayer 26 is etched until the n-type cladding layer 25 or thehigh-temperature buffer layer 24 is exposed as shown in FIG. 1( d). Asthus etched surface determines the end surfaces of the semiconductorlight emitting device. The etching process is designed so that the lightemitting surfaces exposing the active layer and end surfaces facing tothe light emitting surface through a light waveguide are (0001) crystalplane of the Ga_(n)In_(1−n)N layer. Accordingly, the end surfaces arefinished to be smooth and favorable quality mirror surfaces. Then, apattern of metal coating made of Au or Al is provided to form a p-sideelectrode 29 on the cap layer 28 and an n-side electrode 30 on theexposed high-temperature buffer layer 24 (FIG. 1( e)). This is followedby dicing to produce an LED chip form shown in FIG. 2. Alternatively,the cap layer 28 and a part of the upper cladding layer 27 are etchedwith p-side electrode being used as a mask by a reactive ion etchingprocess under an atmosphere of Cl₂ gas to form a mesa type, then thesubstrate 21 is cleft in such a direction that smooth crystal planes ofthe substrate and epitaxial growth layers are exposed. As a result, thesemiconductor laser chip of Example 1 has a light emitting surfacehighly smoothed while its p-side electrode 29 extending in a stripe of 4to 10 micrometers wide and provided with mirror end surfaces (FIG. 1(f)).

The insulating layer of Example 1 for covering the gallium nitride typecompound semiconductor layer is made of silicon nitride (for exampleSi₃N₄) which can easily be processed only by means of simple heattreatment under an atmosphere of nitrogen gas. Also, nitrogen atomsexist commonly in both the insulating layer and the gallium nitride typecompound semiconductor layer, thus allowing any interface between thelayers to be matched as free from undesirable distortion, accordinglythe gallium nitride type compound semiconductor layer with high qualitysurface can be obtained.

EXAMPLE 2

This Example 2 shows an example according to the first aspect of thepresent invention wherein alminum oxide is employed as an insulatinglayer 22 instead of sillicon nitride and others are the same as inExample 1. As is the case with Example 1, preparing a single-crystalsilicon substrate having (111) crystal plane as a principal plane, aninsulating layer 22 comprising aluminum oxide, Al₂O₃, instead of siliconnitride is formed by means of sputtering, evaporation or the like. Theinsulating layer is formed by a thickness of approximately 0.01 to 0.1micrometer over a prepared single-crystal silicon substrate 21.Similarly, as is the case with Example 1, a couple of buffer layers, acouple of cladding layers, and an active layer of gallium nitride typecompound semiconductor materials by means of MOCVD method are stackedand then semiconductor light emitting device shown in FIGS. 1( a) to1(f) or FIG. 2 is obtained.

In this Example 2, the gallium nitride type compound semiconductorlayers, an aluminum oxide intervening between the gallium nitride typecompound semiconductor layers and the single-crystal sillicon substrate,allows their properties to remain stable. Accordingly, thecharacteristic properties of the layers are determined by a wider marginallowing more freedom of designing the light emitting device.

In each of the prescribed Examples 1 and 2, both the light emittingsurfaces of the both sides of the active layer 26 are substantiallyperpendicular to the top surface of the substrate 21. Also, the activelayer 26 is sandwiched by the two, upper cladding layer and lowercladding layer which are high in band gap energy. As a result, thesemiconductor laser acts as a resonator provided with two oppositemirror end surfaces thus increasing the efficiency of light emission. Alaser beam output from the active layer 26 proceeds parallel to thesurface of the substrate and a direction of the laser beam can easily bealigned with the optical axis of a converging lens.

In the prescribed Examples 1 and 2, the light emitting end surfaces maybe produced by cleavage instead of etching. In case of the cleavage, thestacked semiconductor layers structure is cleft in a directionperpendicular to the etching direction which is determined for formingthe n-side electrode 30. Accordingly, while its cleft surfaces includethe light emitting surfaces, a resultant device acts as an opticalresonator provided with mirror surfaces which are highly finished.

EXAMPLE 3

This Example 3 shows an example according to the second aspect of thepresent invention wherein gallium nitride type compound semiconductorlayers including a light emitting layer are stacked to product on agallium nitride type semiconductor substrate as a semiconductorsubstrate. In this case, since the gallium nitride type compoundsemiconductor cannot directly be produced, the gallium nitride typecompound semiconductor is obtained by providing the gallium nitridecompound semiconductor layers on the other semiconductor substrate,utilizing the semiconductor layers as a new substrate, so that asemiconductor for blue light emitting element comprising gallium nitridetype compound semiconductor is realized.

As shown in FIG. 3( a), a low-temperature buffer layer 2 and ahigh-temperature buffer layer 3 both comprising a gallium nitride typecompound semiconductor material are first deposited on a single-crystalsemiconductor substrate 1 by use of MOCVD method.

The single-crystal semiconductor substrate 1 can be selected from thegroup consisting of GaAs single crystal semiconductor, GaPsingle-crystal semiconductor, InP single-crystal semiconductor, and Sisingle-crystal semiconductor, each having a (111) crystal plane as acrystal plane. The single-crystal semiconductor substrate 1 having (111)crystal plane is substantially used because crystalline quality of thegallium nitride type compound semiconductor layer is most suitable.Also, above-mentioned single-crystal semiconductor substrate, forexample GaAs and the like, is selected as its lattice constant is moreapproximate to that of gallium nitride type compound semiconductors thanany other materials, thus minimizing distortion on the gallium nitridetype compound semiconductor layers.

For deposition of an Al_(p)Ga_(q)In_(1−p−q)N (0≦p<1, 0<q≦1, 0<p+q≦1)semiconductor layer, the MOCVD method may be implemented by placing thesubstrate in a reaction furnace and feeding carrier gases in each gasflow rate with a desired ratio of a carrier of H₂ gas, a raw gas of Alin the form of TMA as metal organic gas, a raw gas of Ga in the form ofTMG as metal organic gas, a raw gas of In in the form of TMI as metalorganic gas, and N in the form of NH₃ gas for chemical reaction over thesubstrate. If the semiconductor layers of different compositions arewanted to be grown, different compositions can be deposited by varyingthe ratio of the materials or adding relevant elements of gaseous form.The Al_(x)Ga_(1−x)N layer and the Ga_(n)In_(1−n)N layer in Example 1 areparticular examples in case of the Al_(p)Ga_(q)In_(1−p−q)N layer at p=x,q=1−x and p=o, q=n.

The single-crystal structure of the gallium nitride type compoundsemiconductor is developed by heating at 700° C. to 1200° C. It is notedthat the crystal direction may not completely be uniform if the layer isdeposited directly on the substrate which is different material inlattice constant and the like from that of the layer. It is thus desiredto form the low-temperature buffer layer 2 of polycrystalline form byapproximately 0.01 to 0.2 micrometer thick, at low temperature rangingfrom 400° C. to 700° C., and to deposit the high-temperature bufferlayer 3 of approximately 50 to 200 micrometers in thickness on the layer2 at high temperature ranging from 700° C. to 1200° C. Thelow-temperature buffer layer 2 is shifted from polycrystalline form tosingle-crystal form during th deposition of the high-temperature bufferlayer 3 so that the two layers 2 and 3 are matched to each other inlattice constant.

The single-crystal semiconductor substrate 1 and the low-temperaturebuffer layer 2 are then removed by abrading mechanically or chemicallyat their rear surface side, as shown in FIG. 3( b). The mechanicalabrading may be carried out with a diamond powder grinder and thechemical abrading may be implemented by use of a liquid mixture ofsulfric acid and hydrogen peroxide.

As shown in FIG. 3( c), the high-temperature buffer layer 3 of galliumnitride type compound semiconductor is placed in the reaction furnace asa new substrate and a low-temperature buffer layer 4 with a thickness ofapproximately 0.01 to 0.2 micrometer and a high-temperature buffer layer5 with a thickness approximately 1 to 40 micrometers both which comprisethe single-crystal gallium nitride type compound semiconductor aredeposited on the buffer layer 3 in the prescribed manner. A claddinglayer and an active layer of gallium nitride type compound semiconductormay be deposited directly on the single-crystal semiconductor bufferlayer 3 without forming the two buffer layers 4 and 5, since in chemicalcomposition the buffer layer 3 is similar to the cladding layer andactive layer. The gallium nitride type compound semiconductor layer 3which is utilized as the new substrate is initially formed on thesingle-crystal substrate 1 which is different in composition and mayhave lattice defect or dislocation caused by the lattice mismatch.Possibly, such lattice defect and dislocation are easily propagated tothe single-crystal gallium nitride type compound semiconductor layersformed on the new substrate. For preventing the propagation, it ispreferable that the two buffer layers 4 and 5 are prepared. The methodof deposition and properties of the two buffer layers 4 and 5 areidentical to those of the low-temperature buffer layer 2 and thehigh-temperature (gallium nitride type compound semiconductor substrate)buffer layer 3 shown in FIG. 3( a).

This process is followed by depositing in sequence an n-type claddinglayer 6, a non-doped or an n-type or a p-type active layer 7, a p-typecladding layer 8, and a cap layer 9 as shown in FIG. 3( d). Each of thecladding layers 6 and 8 has commonly thickness of approximately 0.1 to 2micrometers while the active layer 7 is as thin as approximately 0.02 to0.2 micrometer. The thickness of the active layer 7 may be minimized soas to be free from the generation of lattice defect or dislocation. Onthe contrary, the thickness of the cladding layers 6 and 8 is limitedfor thinning. Accordingly, the cladding layers 6 and 8 are not small inthickness and susceptible to distortion if different materials areemployed for the cladding layer and the buffer layer which are thickerthan the other layers. It is thus preferred that the materialcomposition of the cladding layers 6 and 8 are identical to that of thehigh-temperature buffer layer 5.

For producing the semiconductor layer such as the prescribed claddinglayer and the like, to be n-type semiconductor layer, an impurity dopantof Si, Ge, or Sn is mixed with the gaseous materials in the form ofSiH₄, GeH₄, or SnH₄ respectively. Similarly, when Mg or Zn is added inthe form of CP₂Mg or DMZn metal organic gas to the gaseous materials,the p-type semiconductor layer is deposited. The p-type layer is loweredin resistance by annealing at 400° C. to 800° C. and providing aprotective coating comprising SiO_(z) and the like over the cap layer 9or by exposing to a beam of electrons for separation of H from Mg andfor mobility of Mg (which has been bonded to if of the H₂ as a carriergas or NH₃ as a reaction gas.)

In this example, a double heterojunction is constructed by sandwichingthe active layer 7 between the two, p-type and n-type, cladding layers6, 8. The cladding layers 6 and 8 are arranged to be greater in band gapenergy than the active layer 7. It is known for having a higher rate ofband gap energy to increase p of Al_(p)Ga_(q)In_(1−p−q)N whiledecreasing 1−p−q. Since the two cladding layers 6 and 8 having such ahigh rate of the band gap energy are joined in the sandwich structure,carriers fed into the active layer (which acts as a light emittinglayer) can be trapped between the two energy barrier produced betweenthe active layer and the cladding layer. Accordingly, the action ofemission and recombination will be enhanced distinctly in the structureof the present invention as compared with a conventional homojunctionstructure where pn junction is developed between two equal materials,thus allowing the efficiency of emission of light to be increased. Themethod of producing light emitting device according to the presentinvention is not limited to the double heterojunction structure but alsoapplicable with equal success to a semiconductor multilayered devicewhich has a homojunction or heterojunction structure for pn junctionwith only their layer compositions being varied. By providing a stripegroove by semiconductor laser, similarly the semiconductor lightemitting device having a refractive index type waveguide constructioncan be produced. It is noted that the cap layer 9 is provided forminimizing the contact resistance on an electrode metal 10 and has athickness of at most approximately 0.2 to 2 micrometers.

Next, as described above, the surface of the semiconductor layers iscoated with a protective layer made of SiO₂, Si₃N₄, Al₂O₃, GaAs or InPand annealed for approximately 20 to 60 minutes at a temperature rangingfrom 400° C. to 800° C. Alternatively, it is exposed to a beam ofelectrons with approximately 3 to 20 kV of an accelerating voltage whileno protective layer is applied. As a result, the bonding between Mg andH which are doped in the p-type layer is canceled and activated thusdecreasing the electrical resistance of the p-type layer. As describedabove a semiconductor wafer having a multilayer structure for asemiconductor light emitting device is produced.

A lower electrode 11 (at n side) is provided on the rear side of thesemiconductor wafer by means of vapor deposition or sputtering of Au orAl material. On the front side of the semiconductor wafer, an upperelectrode 10 (at p side) is arranged by patterning at a central regionin order to give a space for the emission of light when the element isemployed for an LED, or in order to restrict a current receiving areawhen the element is employed for a semiconductor laser diode. Thesemiconductor wafer is finally cleft to a plurality of semiconductorlight emitting chips, one being shown in a perspective view of FIG. 3(e).

To form an LED, the semiconductor light emitting chip is placed in alead frame, connected by wire bonding, and molded with epoxy resinmaterial. The semiconductor light emitting chip turns to a laser diodewhen it is placed on a stem framework, connected by wire bonding, andsealed with a cap.

The light emitting device and the method for producing the sameaccording to the second aspect of the present invention will now beexplained in more details referring to Examples 4 to 6.

EXAMPLE 4

FIG. 4 is an explanatory cross sectional view of a gallium nitride typedouble heterojunction LED produced by the method of the Example 3. Thedouble heterojunction is produced by varying a ratio of Al, Ga, and Inof a gallium nitride type compound semiconductor material expressed byAl_(p)Ga_(q)In_(1−p−q)N.

A low-temperature buffer layer 4 comprising n-typeAl_(v)Ga_(w)In_(1−v−w)N (where 0≦v<1, 0<w≦1, 0<v+w≦1, v≦x, and1−x−y≦1−v−w), of approximately 0.01 to 0.2 micrometer thick, isdeposited at a low temperature of 400° C. to 700° C. by MOCVD method ona gallium nitride type compound semiconductor substrate 3 which has athickness of 50 to 200 micrometers and comprises an n-typeAl_(v)Ga_(w)In_(1−v−w)N semiconductor layer, with utilizing the galliumnitride type compound semiconductor layer as a new substrate as shown inFIG. 3( b). Then, a high-temperature buffer layer 5 comprising n-typeAl_(v)Ga_(w)In_(1−v−w)N (for example, v=0, w=1) which is identical tothe composition of the gallium nitride type compound semiconductorsubstrate 3 and having a thickness of approximately 1 to 40 micrometersis provided on the buffer layer 4 at a high temperature of 700° C. to1200° C. by the same MOCVD method. This is followed by growing an n-typecladding layer 6 comprising n-type Al_(x)Ga_(y)In_(1−x−y)N (where 0≦x<1,0<y≦1, 0<x+y≦1, for example y=1−x) to a thickness of approximately 0.1to 2 micrometers at 700° C. to 1200°, an active layer 7 comprisingnon-doped Al_(m)Ga_(n)In_(1−m−n)N (where 0≦m<1, 0<n≦1, 0<m+n≦1, m<x, and1−m−n>1−x−y, for example m=o) to a thickness of approximately 0.02 to0.2 micrometer, and a p-type cladding layer 8 comprising p-typeAl_(x)Ga_(y)In_(1−x−y)N to a thickness of approximately 0.1 to 2micrometers. Furthermore, a cap layer 9 comprisingAl_(r)Ga_(s)In_(1−r−s)N (where 0≦r<1, 0<s≦1, 0<r+s≦1, r≦x and1−x−y≦1−r−s, for example r=0, s=1) and having a thickness of aboutapproximately 0.2 to 2 micrometer is deposited on the p-type claddinglayer 8 to reduce contact resistance at the electrode.

In the stacked layers the two cladding layers 6 and 8 are identical incomposition to each other and greater in band gap energy than the activelayer 7. That is, by increasing the amount of Al and decreasing theamount of In, the band gap energy of the material is increased. As theactive layer 7 having a smaller band gap energy is sandwiched betweenthe higher band gap energy cladding layers 6 and 8, carriers fed intothe active layer 7 are trapped between the energy barriers, thusincreasing the efficiency of light emission.

The semiconductor layers are then exposed to a beam of electrons forminimizing the resistance in the p-type layer 8, equipped withelectrodes, whereby obtaining a semiconductor wafer having multi-layerstructure and the semiconductor wafer is cleft to specific chips. As aresult, an LED for blue light of which the output is approximately 0.5candela (cd) in luminance having a double heterojunction is obtained.

The LED of Example 4 has the low band gap energy active layer sandwichedto have a double heterojunction and will thus be increased in theefficiency of light emission. Simultaneously, as thicker layers, thecladding layers and the buffer layers comprising the semiconductormaterials which are indentical in chemical composition, while the othersemiconductor layers made of different compositions are reduced to suchsmaller values of the thickness as not to create any lattice defect.Accordingly, the semiconductor wafer having multilayer structure isimproved in the physical quality and will thus be cleft with much ease.

EXAMPLE 5

A semiconductor laser device, whose explanatory cross sectional view isshown in FIG. 5, of this example is identical to the LED of Example 4 bythe fact that the layers and electrodes are produced by the same manner.Hence, like components shown in FIG. 5 are denoted by the same numeralsas those of Example 4. Stacking each semiconductor layer, as is the casewith Example 4, the laser device of Example 5 has a mesa type producedon the top thereof by etching a cap layer 9 at both sides of an upperelectrode 10, and upper side, of a p-type cladding layer 8. Thisarrangement allows a current to flow across a central region of theactive layer. Also, both end surfaces of the laser are mirror surfacesby cleaving so that oscillation occurs between the mirror end surfaces.As a result, the semiconductor laser device for blue light of which theoutput is approximately 0.2 mW is obtained.

EXAMPLE 6

An LED of this Example 6, of which explanatory cross sectional view isshown in FIG. 6, having a pn junction is produced by growing alow-temperature buffer layer 14 comprising n-type GaN semiconductor by athickness of approximately 0.01 to 0.2 micrometer on a gallium nitridetype compound semiconductor substrate 13 and a high-temperature bufferlayer 15 comprising n-type GaN semiconductor by a thickness ofapproximately 1 to 40 micrometers under the same conditions as inExample 4. This is followed by forming an n-type layer 16 comprisingn-type Al_(t)Gal_(1−t)N (0≦t<1), approximately 0.1 to 2 micrometersthick, a p-type layer 17 comprising p-type In_(u)Ga_(1−u)N (0≦u<1),approximately 0.01 to 2 micrometers thick, and a cap layer 18 comprisingp-type: Al_(z)Ga_(1−z)N (0≦z<1). The p-type layers are then exposed to abeam of electrons produced with an accelerating voltage of approximately3 kV to 20 kV and are annealed. Then a lower electrode 11 (at n side)and an upper electrode 10 (at p side) are provided, the LED having a pnheterojunction is obtained. While its heterojunction enhances theefficiency of light emission, an LED for blue light of which theluminance is approximately 0.2 candela is obtained.

EXAMPLE 7

This Example 7 is an example according to the third aspect of thepresent invention using group II-VI compound material, for example,ZnSe, ZnS as a semiconductor substrate.

First, group II-VI compound semiconductor substrate as the substrate 3,such as a ZnSe substrate and a ZnS substrate, approximately 50 to 500micrometers thick, instead of a gallium nitride type semiconductorsubstrate is prepared, as shown in FIG. 3( b).

Either ZnSe or ZnS of the substrate is preferably provided with a (111)crystal plane as a principal plane. Also, the group VI atoms of thegroup II-VI compound semiconductor substrate, namely Se or S, areexposed on the top surface. The method for producing the substrate is asfollows. First, determining the direction of (111) crystal plane isdetermined by use of an X-ray diffraction analysis method. The surfaceswhere the VI group atoms are exposed (hereinafter referred to as “A”plane) are orientated in one direction upon dicing so that the (111)crystal plane is a principal plane. This allows the (111) crystal planeor A plane to be identified when a larger atom-radius is measured with atunnel current microscope since a direction where atom-radius is largeris (111) A plane.

Then, in the same manner as those of Examples 3, 4, shown in FIG. 3( c)to FIG. 3( e), a low-temperature buffer layer, Al_(v)Ga_(w)In_(1−v−w)N(where 0≦v<1, 0<w≦1, 0<v+w≦1, for example v=0, w=1) and othersemiconductor layers are stacked and the electrodes 10, 11 are formed.As a result, the semiconductor light emitting device of Example 7 whichhas the same sectional structure as shown in FIG. 4 is obtained.

In Example 7, if the cladding layer and the active layer are directlygrown to gallium nitride type compound single crystal structures on thesemiconductor substrate, lattice defect and dislocation caused bylattice mismatch may occur. This problem is avoided by providing thelow-temperature buffer layer 4 and high-temperature buffer layers 5 toobtain the cladding layer and the active layer, these layers having finecrystalline structure. The high-temperature buffer layer 5 may beutilized also as a part of the cladding layer.

Further in Example 7, since semiconductor substrate is not made by meansof epitaxial growth, desired thickness can be obtained. Thus thehigh-temperature buffer layer need not to be so thick, and can beprovided sufficiently by approximately 2 to 5 micrometers thick.

The semiconductor layers are then exposed to a beam of electrons forminimizing the resistance in the p-type layer 8, equipped withelectrodes 10, 11 whereby obtaining a semiconductor wafer havingmultilayer structure, and the semiconductor wafer is cleft to a specificform. As a result, an LED for blue light of which the luminance isapproximately 0.5 candela (cd) having a double heterojunction isobtained.

The LED of Example 7 has the low band gap energy active layer sandwichedto have a double heterojunction and will thus be increased in efficiencyof light emission. Simultaneously, the cladding layers and the bufferlayers comprising the thick semiconductor materials which are identicalin chemical composition while the other semiconductor layers made ofdifferent compositions are reduced to such smaller values of thethickness as not to generate any lattice defect. Accordingly, thesemiconductor wafer having multilayer structure is improved in thephysical quality and will thus be cleft with much ease.

EXAMPLE 8

This Example 8 shows an example wherein a semiconductor substrate whichis the same substrate as Example 7, is employed as a substrate, and asemiconductor light emitting device is employed as a semiconductorlaser.

In this example, since the same type semiconductor substrate as that ofExample 7 is prepared, stacking the semiconductor layers same as Example5 are stacked and the electrodes are formed, the semiconductor laserhaving the same structure is obtained as shown in FIG. 5.

This arrangement allows a current to flow across a central region of theactive layer. Also, both end surfaces of the laser are mirror surfacefinished by precision cleaving so that oscillation occurs between themirror end surfaces. As a result, the semiconductor laser device forblue light of which the output is approximately 0.2 mW is obtained.

EXAMPLE 9

This Example 9 shows an example wherein a II-VI compound semiconductorsubstrate is employed as a semiconductor substrate, which is the same asExample 7, and an LED having a pn junction is employed as asemiconductor light emitting device.

In this Example 9, since the same type semiconductor substrate as inExample 7 is prepared, the semiconductor layers same as Example 6 arestacked and the electrodes is formed, then the pn junction LED having aheterojunction having the same structure as shown in FIG. 6 is obtained.

While its heterojunction enhances the efficiency of light emission thanhomojunction, the LED for blue light of which the luminance is at least100 millicadelas (mcd) is obtained.

EXAMPLE 10

This Example 10 shows an example according to the fourth aspect of thepresent invention, wherein a group III-V compound semiconductorsubstrate such as GaAs, InAs, GaP and InP as a semiconductor substrateis employed.

First, a semiconductor substrate comprising the group III-V compoundmaterial such as GaAs, InAs, GaP or InP, approximately 50 to 500micrometers thick is employed instead of the gallium nitride typesemiconductor substrate as shown in FIG. 3( b). The substrate isprovided with a (111) crystal plane as a principal plane. Also, group Vatoms, namely As or P, are exposed on the top surface (hereinafterreferred to as “(111) A plane”). The method for producing the substratestarts with determining the direction of (111) crystal plane by use ofan X-ray diffraction analysis method. After dicing in parallel with the(111) crystal plane, a surface containing atoms of a larger radius isthe (111) A plane. Hence, when the larger radius is measured by a tunnelcurrent microscope, its surface can be identified as the (111) A plane.

Then, in the same manner as those of Examples 3, 4, shown in FIG. 3( c)to FIG. 3( e), a low-temperature buffer layer, Al_(v)Ga_(w)In_(1−v−w)N(where 0≦v<1, 0<w≦1, 0<v+w≦1, for example v=0, w=1) and othersemiconductor layers are stacked and the electrodes 10, 11 are formed.As a result, the semiconductor light emitting device of Example 10 whichhas the same sectional structure as shown in FIG. 4 is obtained.

If the cladding layer and the active layer are directly grown to galliumnitride type compound single-crystal structures, lattice defect anddislocation caused by lattice mismatch may occur. This problem isavoided by providing the low-temperature buffer layer 4 andhigh-temperature buffer layers 5. The high temperature buffer layer 5may be utilized as a part of the cladding layer.

In this Example 10, like other examples, since the semiconductorsubstrate is not the semiconductor layer substrate which is made bymeans of epitaxial growth, the substrate is thick enough. Thus thehigh-temperature buffer layer needs not to be so thick, and can beprovided sufficiently by approximately 2 to 5 micrometers thick.

After forming the semiconductor layers, the semiconductor layers is thenexposed to a beam of electrons for minimizing the resistance in thep-type layer, equipped with electrodes, whereby obtaining asemiconductor wafer having multi-layer structure and the semiconductorwafer is deft to a specific form. As a result, an LED for blue light ofwhich the luminance is approximately 0.5 candela (cd) having a doubleheterojunction is obtained.

The LED of Example 10 has the low band gap energy active layer which issandwiched to compose a double heterojunction and will thus be increasedin efficiency of light emission. Simultaneously, the cladding layer andbuffer layer comprising the semiconductor materials which are equal toeach other in chemical composition, while the other semiconductor layersmade of different composition are reduced to such smaller values of thethickness as not to generate any lattice defect. Accordingly, thesemiconductor wafer having multilayer structure is improved in physicalquality and will thus be cleft with much ease.

EXAMPLE 11

This Example 11 shows an example wherein a group III-V compoundsemiconductor substrate which is the same substrate as in Example 10 isemployed and a semiconductor laser is employed as a light emittingdevice. In this Example 11, since preparing the same semiconductorsubstrate as Example 10, stacking the semiconductor layers same asExample 5 and forming the electrodes, a semiconductor laser can beobtained that has the same structure shown in FIG. 5.

This arrangement allows a current to flow across a central region of theactive layer. Also, both end surfaces of the laser are mirror surfacesby cleaving so that oscillation occurs between the mirror ends surfaces.As a result, the semiconductor laser device of blue light of which theoutput is approximately 0.2 mW is obtained.

EXAMPLE 12

This Example 12 shows an example wherein a semiconductor substrate ofgroup III-V compound material which is the same substrate as in Example10 is employed and a pn junction LED is employed as a light emittingdevice. In this Example 12, since preparing the same semiconductorsubstrate as Example 10, stacking the semiconductor layers same asExample 6 and forming the electrodes, a pn junction LED having aheterojunction can be obtained that has the same structure as in FIG. 6.

While its pn heterojunction ensures a higher efficiency in the lightemission than pn homojunction, the LED blue light of which the luminanceis approximately 0.2 candela (cd) is obtained.

Although the foregoing examples according to the present invention aredescribed in the form of semiconductor laser devices having a stripepattern of the current feeding region produced by mesa etchingtechnique, they are illustrative but not limitative. Other variations ofthe blue light semiconductor laser device of gallium nitride typecompound semiconductor having a current blocking layer of oppositeconductive type, the blocking layer being inserted in the cladding layerin which a stripe groove is formed, or having an embedded structure willbe made with equal success wherein the sides are perpendicular to thesubstrate. The gallium nitride type compound semiconductor layers arenot limited to the compositions shown above. It may be determined withpand q of Al_(p)Ga_(q)In_(1−p−q)N (where 0≦p<1, 0<q≦1 and 0<p+q≦1) sothat the band gap energy of the active layer is smaller than that of thecladding layers. Also, a material wherein a part or all of N inAl_(p)Ga_(q)In_(1−p−q)N is substituted with As and/or P is applicable tothe present invention. Not only the laser diode but also the LEDaccording to the present invention can be adapted in which a laseroutput is emitted from not a top but a perpendicular side. Thus, thelaser output of the LED is allowed to emit in a specified directionregardless of either the double heterojunction or the pn junctionstructure.

According to the first aspect of the present invention, thesingle-crystal substrate is employed and gallium nitride type compoundsemiconductor layers are grown on a thin insulating layer. This allowsdistortion at the interface to be minimized as compared with directdeposition of the gallium nitride type compound semiconductor layersover a conventional sapphire substrate. The single-crystal substrate isinexpensive and advantageous in machining process and will thusfacilitate the production of devices.

The multilayer structure is composed mainly of the thick gallium nitridetype compound semiconductor layers including cladding layers and thelike and the insulating single-crystal silicon substrate, and will becleft with ease having its light emitting surfaces mirror surfaces. Thisallows favorable production of semiconductor lasers for blue light.

According to the second to fourth aspects of the present invention, thesubstrate is not made of an insulating material and allows lowerelectrodes to be attached directly thereto. It is unnecessary to carryout a conventional etching process to expose corresponding regions of aconductive layer as the electrodes. Since such a dry etching process iseliminated, the overall procedure is simplified and deterioration causedby the resistance in the material quality caused by contamination duringthe etching process will be prevented.

Also, the semiconductor substrate is made of a gallium nitride typecompound semiconductor similar to those of the relatively thick layerssuch as the cladding layers. As a result, the substrate and the layersare matched in crystal alignment and can thus be cleft with ease tospecific mirror end surfaces. This allows production of semiconductorlasers for blue light.

Since the substrate is made of a gallium nitride type compoundsemiconductor which is the same kind as the operating layers. Hence, thesubstrate and the active layer are equal in the lattice constant thusproducing lattice matching and minimizing lattice defect or dislocation.The quality of the semiconductor layers of the device will thus beenhanced by improving the efficiency of light emission and increasingthe operation life.

According to the fourth aspect of the present invention, either ZnSe orZnS of the substrate allows transmission of light without absorption oflight of at least 470 nanometers wavelength or at least 320 nanometerswavelength respectively. When the light emitting device of the presentinvention produces light of which the wavelength is at mostabove-mentioned value, its operational efficiency will be particularlyoptimum.

By use of group III-V compound semiconductor material as a semiconductorsubstrate will be improved in performance when the substrate is made ofgallium arsenide with the buffer layers containing gallium, of indiumarsenide with the buffer layers containing indium, of gallium phosphidewith the wavelength of at least 550 nanometers or the buffer layerscontaining gallium, or of indium phosphide with the buffer layerscontaining indium.

By use of group II-VI or group II-V compound material as thesemiconductor substrate is employed to match in lattice constant ofcrystal structure, undesirable lattice defect or dislocation will beprevented. As a result, the quality of the semiconductor layers isenhanced thus improving the efficiency of light emission and increasingthe operational life of the semiconductor light emitting device. Thesemiconductor layers can also be cleft with much ease giving fine mirrorsurfaces and implementing favorable semiconductor laser for blue light.

Though several embodiments of the present invention are described above,it is to be understood that the present invention is not limited only tothe above-mentioned, various changes and modifications may be made inthe invention without departing from the sprit and scope thereof.

1. A method for producing a semiconductor light emitting devicecomprising the steps of: (a) forming an insulating layer on asingle-crystal silicon substrate; (b) forming a gallium nitride typecompound semiconductor layer as a buffer layer on the insulating layer;(c) stacking on the buffer layer in sequence a lower cladding layer, anactive layer, an upper cladding layer, and a cap layer, these layersbeing made of the gallium nitride type compound semiconductor; (d)exposing a predetermined surface of the buffer layer by etchingperpendicularly to the single-crystal silicon substrate; (e) formingelectrodes on both the cap layer and the predetermined surface of thebuffer layer exposed by the etching treatment in step (d), wherebyobtaining a semiconductor wafer having multilayer structure; and (f)separating the semiconductor wafer to chips by dicing or by cleaving,wherein the step of forming the insulating layer is implemented byremoving an oxide film over the single-crystal silicon substrate andforming a silicon nitride layer by heating under an atmosphere ofnitrogen gas.
 2. The method for producing the semiconductor lightemitting device of claim 1, wherein the single-crystal silicon substratehas a (111) crystal plane as a principal plane.
 3. A method forproducing a semiconductor light emitting device comprising the steps of:(a) forming an insulating layer on a single-crystal silicon substrate;(b) forming a gallium nitride type compound semiconductor layer as abuffer layer on the insulating layer; (c) stacking on the buffer layerin sequence a lower cladding layer, an active layer, an upper claddinglayer, and a cap layer, these layers being made of the gallium nitridetype compound semiconductor; (d) exposing a predetermined surface of thebuffer layer by etching perpendicularly to the single-crystal siliconsubstrate; (e) forming electrodes on both the cap layer and thepredetermined surface of the buffer layer exposed by the etchingtreatment in step (d), whereby obtaining a semiconductor wafer havingmultilayer structure; and (f) separating the semiconductor wafer tochips by dicing or by cleaving, wherein the step of forming theinsulating layer is implemented by growing a layer of aluminum oxide. 4.A method for producing a semiconductor light emitting device comprisingthe steps of: (j) preparing a group II-VI compound semiconductorsubstrate; (k) stacking a buffer layer of gallium nitride type compoundsemiconductor on a principal plane of the group II-VI compoundsemiconductor substrate; (l) stacking on the buffer layer in sequence alower cladding layer, an active layer, an upper cladding layer, and acap layer, these layers being made of gallium nitride semiconductor,with matching crystal lattice of each layer to one another; (m) formingelectrodes on both the top of the cap layer and the bottom of the groupII-VI compound semiconductor substrate, whereby obtaining asemiconductor wafer having multilayer structure; and (n) cleaving thesemiconductor wafer to chips.
 5. The method for producing asemiconductor light emitting device of claim 4, wherein the step ofstacking the buffer layer is implemented by forming a low-temperaturebuffer layer at low temperature and then, by forming a high-temperaturebuffer layer at high temperature.
 6. The method for producing asemiconductor light emitting device of claim 4, wherein the bufferlayers are made of n-type GaN, the lower cladding layer is made ofn-type Al_(x)Ga_(1−x)N (0<x<1), the active layer is made ofGa_(n)In_(1−n)N (0<n≦1), the upper cladding layer is made of p-typeAl_(x)Ga_(1−x)N (0<x<1), and the cap layer is made of p-type GaN.
 7. Themethod for producing a semiconductor light emitting device of claim 4,wherein the group II-VI compound semiconductor substrate having aprincipal plane, the principal plane being a top surface comprisinggroup VI atoms of the group II-VI compound semiconductor substrate, isprepared.
 8. A method for producing a semiconductor light emittingdevice comprising the steps of: (o) preparing a group III-V compoundsemiconductor substrate, said group III-V compound semiconductorselected from the group consisting of GaAs, InAs, GaP, and InP; (p)stacking a buffer layer of gallium nitride type compound semiconductoron a principal plane of the group III-V compound semiconductorsubstrate; (q) stacking on the buffer layers in sequence a lowercladding layer, an active layer, an upper cladding layer, and a caplayer, these layers being made of gallium nitride type semiconductor,with matching crystal lattice of each layer to one another; (r) formingelectrodes on both the top of the cap layer and the bottom of the groupIII-V compound semiconductor substrate, whereby obtaining asemiconductor wafer having multilayer structure; and (s) cleaving thesemiconductor wafer to chips.
 9. The method for producing semiconductorlight emitting device of claim 8, wherein the step of forming the bufferlayers is implemented by forming a low-temperature buffer layer at lowtemperature and then, by forming a high-temperature buffer layer at hightemperature.
 10. The method for producing the semiconductor lightemitting device of claim 8, wherein the group III-V compoundsemiconductor substrate having a principal plane, the principal planebeing a top surface comprising group V atoms of the group III-V compoundsemiconductor substrate, is prepared.